The invention relates to gas turbine helicopter engines.
As a result of the efforts made to reduce the noise generated by the rotation of helicopter rotor blades, the applicant found that the noise actually from gas turbine engines is becoming a significant component of the helicopters' overall sound level.
Therefore, an appreciable supplementary lowering of this sound level could be obtained by silencing the gas turbine engines themselves.
For that purpose, it is proposed in EP 1 010 884 to provide the walls of a multichannel nozzle receiving gas issuing from the turbine with a coating capable of absorbing sound energy. In view of the sound frequencies to be attenuated, the coating is relatively thick, which represents a substantial mass, all the more that the coating must be in a material capable of withstanding the temperature of the primary flow exiting from the turbine. In addition, the thickness of the coating cannot allow the gas flow issuing from the turbine to ensure an efficient aspiration of a possible secondary flow used for cooling the engine compartment through a passage surrounding the nozzle, whereby the acoustic treatment (hush kitting) of the nozzle is not compatible with an ejector function.